The present invention relates generally to an electronic imaging system represented by a video camera or still video camera, and more particularly to a wide-angle image pickup lens designed to reduce the size of such an electronic imaging system by use of a reflective member.
With the recent advent of miniature electronic image pickup devices such as CCDs, imaging units become smaller and smaller. Typically, JP-A 10-39207 shows an imaging unit in which a taking lens is made up of as few as four lens elements with a reduced length, and so only a lens portion is made compact.
JP-A 11-95096 discloses a taking lens made up of as few as four lens elements; however, this taking lens uses an aspheric lens and so costs much. The size of the taking lens itself is reduced by virtue of an image plane size reduction; as shown in FIG. 17, however, the presence of wires and terminals on the outer region of the light-receptive plane of an image pickup device 15 and the package size of the image pickup device 15 make it impossible to reduce size relative to the height direction. It is noted that FIG. 17 is illustrative in section of an imaging unit inclusive of its optical axis. Reference numeral 10 indicates a frame (lens barrel), 11 an image pickup lens, 12 an aperture stop, 13 an optical filter, 14 a cover glass and 15 an image pickup device. Regarding an objective lens for endoscopes, another approach to size reductions in the height direction has been proposed, wherein an optical path is bent to position an image pickup device laterally.
JP-A 9-122070 shows that an optical filter and a triangular prism are located at a back focus portion of a taking lens. However, this is not favorable for the total length of an imaging unit because of the need of a long bending space. Although constraints on the length of an endoscopic objective lens are loose, application of a long lens unit to a camera is not preferable because it means that camera thickness increases.
JP-A 11-109223 shows that a long back focus is ensured; however, the location of an aperture stop between a negative front lens group and a positive rear lens group causes rim rays at the rear lens group to become high, resulting in an increased lens diameter. Accordingly, when a reflective member is located at a back focus portion of this type, rays at the entrance surface of the reflective member become high, leading to the need of increasing the volume of the location where the reflective member is located.